Frequency modulation



April 7, 1.942` E. s. WINLUND 2,279,030 Y FREQUENCY MODULATION Filed May 31, 1940 HG. l.

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l 15 Il 33 E7 ouv'Pu'r 4. SCREEN :inventor Edin/ond 1 l/Vnlund FREQUENCY y Patented Apr. 7,` 1942 FREQUENCY MoDULATIoN Edmond S. Winlund, Merchantville, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application May 31, 1940, Serial No. 337,999

Claims.

This invention relates to frequency modulation systems, and .more particularly to a crystal stabilized frequency modulated oscillator em- .ploying negative feedback to reduce hum and distortion.

It is Well known that considerable diiliculty has been vexperienced in the past in stabilizing the mid-frequency, or unmodulated carrier frequency of a frequency modulated transmitter, particularly at high operating frequencies. It will be appreciated that an oscillator which in itself is inherently stable is unsuited for frequency modulation, since frequency modulation is the opposite of frequency stability. Consequently, it has been proposed to employ low frequency oscillators, which may be made to have a fairly constant average frequency, and to utilize frequency multipliers to obtain the desired output frequency.

Inherent frequency stability is advantageously produced by means of a'quartz crystal resonator, as is well known. Crystal controlled oscillators utilizing zero temperature coemclent crystalsor crystals whose temperature is carefully regulated, are generally unsuited for use in frequency modulated systems since their frequency cannot readily be varied. It is highly desirable, however, to take advantage of the frequency stability which may be achieved by means of a crystal element in a frequency modulated oscillator. One system for accomplishing this end is described in a copending application of Ehret and Barnes, Serial No. 336,550, led May 22, 1940, in which the position of one electrode of the crystal is varied in accordance with the modulating potential. In

such a system, the physical mass of the electrode and other moving parts tends to limit the frequency response which may be obtained and to V reduce the higher frequency components of the modulating signal. In addition, it is necessary to avoid changes in the normal electrode position due to the expansion of the armature with changes in temperature.

It is the principal object of this invention to provide a frequency modulation system which takes advantage of the inherent stability of a crystal resonator and avoids movement of the crystal electrode.

Other objects of this invention include the provision of a stabilized oscillator which may be modulated over a range of frequencies; the provision of a stabilized oscillator to decrease distortion; and the provision of a crystal stabilized oscillator of constant average frequency which may nevertheless be modulated throughout a (Cl. 179--171.5) l y band of frequencies in accordance with a modul lating voltage.

In brief, the foregoing objects are accomplished in accordance with the present invention by connecting the crystal control element to a reactance tube Which is connected in parallel with an oscillator. Thus the crystal element does not control the average oscillatorfrequency directly, but only by means of thereactance tube which is also subject to modulating potentials. The crystal tends to overcome changes in the average frequency of the oscillator due to temperature variations, line voltage variations, and the like,` and at the same time by reducing the effect of the reactance tube, provides a desirable degenerative feedback which reduces-hum and distortion.

The present invention will be better understood from the following description when considered in kconnection with the accompanying drawing, in which Figures 1 and 2 are circuit diagrams of two embodiments of this invention; Figure 3 is the impedance curve of a crystal resonator; and Figure 4 is the equivalent diagram of a crystal resonator.

Referring to Fig. 1, an oscillator tube 5 is connected to an oscillatory tank circuit 'I comprising inductor 9 and capacitor II 'I'he oscillator itself may be a triode, as illustrated, or any other well known type of tube connected in any conventional circuit. The particular tube and oscillator circuit is well known and need not be described in detail.

A reactance tubeV I3 is connected in parallel with the oscillatory circuit 'I. The reactance' tube also may be any known type such as capacitive or inductive, single-ended or pushpull, without ailecting the principle of operation of the present invention. Inthe particular instance illustrated in Fig. 1, the anode and cathode electrodes of the reactance tube I3 are connected in parallel with the frequency controlling circuit of the oscillator. A voltage divider comprising resistors I 5 and I'I and the capacitor I9 is also connected in parallel with the oscillator. The grid of the reactance tube I3 is coupled .to a point intermediate the ends of the voltage divider, for example, to the junction of resistor II and capacitor I9. The grid of reactance tube I3 is also connected to the secondary of a modulation transformer 2i to the primary of which the modulating input voltage is applied. Frequency modulated output is taken from terminals 23 and 25, the former of which is coupled to the high potential terminal of the oscillator. A quartz crystal resonator 2l is connected in parallel with a portion of the voltage crystal, and its electrical equivalent circuit which are illustrated, respectively, in Figs. 3 and 4. In Fig. 4, theequival'ent electrical.'characteristics of the Aquartz crystal itself may beV4 represented by the series connected elements C, L, R corresponding to the capacity, inductance and resistance, respectively, of the crystal., Y q ments in series are effectively inparallel with another capacity C-' which corresponds to the v' circuit and electrode capacities across the crystal element.

Considering the impedance between terminals 2 9 and 3|, illustrated in Fig. 4, at some particular frequency, f I, the capacity C and inductance L are series resonant so that the impedance` is a minimum. At some other frequency,` f2, the shunt capacity C. resonates with the crystal inductance to produce parallel resonance so that the crystal impedancereaches a maximum. At

frequencies below Ithe series resonant frequency fI the crystal is essentially a capacitive reactance since the inductive component becomes negligible. At frequencies above the parallel resonant frequency f2, the shunt impedance ofthe electrode capacity C" effectively short circuits the crystal and the crystal impedance therefore decreases in the manner illustrated.

The operating point of the crystal 21 of Fig. 1 is preferably selected at some point, f3, interme. diate the series resonant frequency fI and the -parallel resonant frequency f2. In this region the change of impedance of the crystal is seen to be maximum. The crystal impedance is effectively in parallel with a portion of the voltage divider from which the energizing voltage of the reac- ,These elerent likewise decreases.

The effect of the crystal controlled element 21 will now be considered. Assuming that the normal frequency of the oscillator is approximately the frequency on the slope of the resonance curve of the crystal, if anything happens to the oscillator which tends to make its frequency increase, it will be appreciated that the impedance of the crystalwill likewise increase. As a result, the attenuation vof the voltage divideris varied andthe amplitude of the radio frequency grid voltage of the reactance tube I3 decreases. As a result, the amplitude of the reactive anode curl 'I'his is equivalent to decreasing the effectiveV shunt inductance produced by the reactance tube,that is, the induc- 'tiveeffectof the reactance tube becomes less.

` This in turn causes the oscillator frequency to tance tube I3 is obtained. Consequently, the attenuation ratio of the voltage divider becomes a function of the oscillator frequency s0 that the amplitude of the radio frequency voltage applied to the grid of the reactance tube is a function of the oscillator frequency.

The function of a reactance tube is well known, but will be explained briefly. When the impedance of the capacitor I9 is small with respect to the impedance of the resistors I5 and I1 the current flowing through the voltage divider is very nearly in'phase with the applied voltage. The applied voltage is the voltage across the oscillatory tank circuit 1. Consequently, the voltage across capacitor I 9, which is applied between grid and cathode of the reactance tube I3, lagscurrent through the capacitor and the oscillatory voltage 1 by nearly 90. This produces in the plate circuit .of the reactance tube a component of current which lags the oscillatory voltage by nearly 90. Such a lagging current is the equivalent ofthe current which would be taken by a predominantly inductive load connected across the oscillator circuit, and, as is well known, an inductance connected across an oscillatory tank circuit tends to increase its frequency. The amplitude of this shunt inductive .current is controlled by the bias which is applied to the reactance tube I3. In the present instance, this bias is an alternating voltage derived from the modulation transformer 2 I. Consequently, upon the application of an alternating voltage to the modulation transformer, the reactance tube I3 varies the frequency of the 0scillator accordingly.

decrease,so that it will be appreciated that the reactance tube by means of the change in the crystal impedance tends to counteract or compensate the original change in the frequency of the oscillator. A similar compensating action occurs when the oscillator frequency tends to decrease for any reason.

It is realized that changes in the oscillator frequency likewise produce a change in the phase of the voltage applied to the reactance tube grid by reason of the change in the phase of the voltage across the crystal element.4 The degree of this phase change, of course, depends upon the relative impedances of the crystal and the shunt resistor. Small changes in phase only produce amplitude modulation and therefore do not effect operation of the reactancetube in controlling the frequency. That is, if the reactance tube plate current varies a small amount from the desired quadrature relation, the plate current merely takes on a resistive component which represents a resistive loss in the reactance tube, and the amplitude of the lreactive component is only slightly changed. The variations of `amplitude can readily be removed by limiting circuits of the type well known to those skilled in the art. Frequency modulation may also be achieved by operating the crystal at a point of maximum phase shift, and deriving a change in the amplitude of the quadrature plate current as a result of the phase change, the change of impedance being minimized.

Fig. 2 is an alternative embodiment of this invention in which the reactive tube I3 is connected to function as a shunt capacity rather than as a shunt inductance.4 The oscillator portion of the circuit is the same as that illustrated in Fig. 1 and need not be described in detail. The voltage divider in the present instance includes resistive and reactive elements, as'before, but in the present case the reactive element, capacitor I9, is a relatively high impedance and is connected between the high terminal of the oscillator and grid while theresistive element 33 is connected between grid and cathode of the reactance tube I3. The current through the yvoltage divider now leads the oscillator voltage because the voltage divider impedance is predominantly capacitive.

The grid voltage of the reactance tube I3 is in phase with thecurrent through resistor 33 and leads the oscillator voltage. Consequently, the resultant anode current is a leading current and corresponds to a shunt capacitance across the aeraoso if the oscillator frequency tends to increase, the crystal impedance likewise increases, as may be seen in Fig. 3. Since the crystal is now connected between grid and cathode, the resultant increase in impedance causes the applied radio frequency grid voltage to increase, thus increasing the plate current and the effective shunt capacitance. As a result, the reactance tube tends to decrease the oscillator frequency and overcome the original change.

It will be appreciated that the control function of the crystal operates not only on changes of frequency produced by oscillator drift, but also by changes produced by the modulating voltage through the reactance tube. This tendency, however, can never produce complete compensation since the crystal only attempts to return the oscillator frequency to its normal value when there is a change in frequency. If the crystall completely compensated for the change produced by the modulating voltage, so that the oscillator was returned to its normal value, it will be appreciated that the impedance change of the crystal would also be reduced to zero and the modulating potential would immediately-tend to change the oscillator frequency again. Consequently, the modulating potential always produces a variation in the oscillator frequency, while the crystal lperforms dual functions of maintaining the average oscillator frequency at a constant value, and providing a desirable degenerative feedback which reduces distortion in the modulating circuit.

Inductor 35, which has been included in series with the voltage divider in the embodiment illustrated in Fig. 2, is not essential to the operation of the circuit, but has been included to adjust the average grid to cathode voltage phase more closely to 90.

While the present invention has been described by means of several specific embodiments, the invention is not limited thereto, and many modiiications may be made. For example, the modulating voltage may be applied to the screen grid of the reactance tube, or the phase shifting may take place in the plate circuit of the reactance tube instead of in the gird circuit, as is well known.

I claim as my invention: l

1. In a frequency lmodulation system, an oscil' lator, a reactance tube coupled to said oscillator for varying the frequencyn thereof in accordance with a modulating potential, and a piezo electric element resonant at a frequency near the frequency of said oscillator in circuit with said reteristic of said tube as a function of the frequency of said oscillator in a direction which tends to partially overcome the change of frequency produced by said modulating potentials.

2. In a frequency modulating system, a source of oscillatory voltage, a reactance tube having input and output circuits, said output circuit being coupled to said source, a voltage divider connected across said source and to said input circuit for applying to said input circuit a voltage substantially in quadrature with said oscillatory voltage, and a frequency responsive elementconnected across a portion 0f said divider.

3. In a frequency modulating system, a source of oscillatory voltage, a reactance tube having cathode, grid and anode electrodes, means coupling said cathode and anode electrodes to said source, a voltage divider including a reactive element connected acrossv said source, sald'grid electrode being coupled to a point on said voltage divider,l and a crystal resonator connected in parallel with a portion of said divider.

4. In a stabilized frequency modulation system, oscillation producing means, a reactance tube having input and output circuits, means coupling said output circuit to saidoscillation producing means, means for applying modulating voltages to said input circuit, a frequency responsive element for deriving a voltage from said oscillations whose amplitude varies as a function of a change in the frequency of said oscillations, means for shifting the phase of said voltage to produce a quadrature voltage, and means for applying said quadrature voltagelto said input circuit so as to tend to compensate for said change.

5. In a stabilized frequency'rmodulation system, oscillation producing means, a reactance tube having input and output circuits, means coupling said output circuit to said oscillation producing means-means for applying modulating voltages to said input circuit, means including a piezof 

